Vertical grating structures placed between a waveguide core and a substrate

ABSTRACT

Structures including a waveguide core and methods of fabricating a structure including a waveguide core. The structure comprises a substrate, a waveguide core, and a grating disposed in a vertical direction between the waveguide core and the substrate. The grating includes a first plurality of layers and a second plurality of layers that alternate in the vertical direction with the first plurality of layers. The first plurality of layers comprise a first material having a first refractive index, and the second plurality of layers comprise a second material having a second refractive index that is greater than the first refractive index.

BACKGROUND

The disclosure relates to photonics chips and, more specifically, tostructures including a waveguide core and methods of fabricating astructure including a waveguide core.

Photonics chips are used in many applications and systems including, butnot limited to, data communication systems and data computation systems.A photonics chip integrates optical components and electronic componentsinto a unified platform. Among other factors, layout area, cost, andoperational overhead may be reduced by the integration of both types ofcomponents on the same chip. Waveguide cores used in photonics chips maysuffer from significant leakage loss of propagating light to thesubstrate, which may degrade performance.

Improved structures including a waveguide core and methods offabricating a structure including a waveguide core are needed.

SUMMARY

In an embodiment of the invention, a structure comprises a substrate, awaveguide core, and a grating disposed in a vertical direction betweenthe waveguide core and the substrate. The grating includes a firstplurality of layers and a second plurality of layers that alternate inthe vertical direction with the first plurality of layers. The firstplurality of layers comprise a first material having a first refractiveindex, and the second plurality of layers comprise a second materialhaving a second refractive index that is greater than the firstrefractive index.

In an embodiment of the invention, a method comprises forming a gratingon a substrate and forming a waveguide core over the grating in avertical direction. The grating includes a first plurality of layers anda second plurality of layers that alternate in the vertical directionwith the first plurality of layers. The first plurality of layerscomprise a first material having a first refractive index, and thesecond plurality of layers comprise a second material having a secondrefractive index that is greater than the first refractive index.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate various embodiments of theinvention and, together with a general description of the inventiongiven above and the detailed description of the embodiments given below,serve to explain the embodiments of the invention. In the drawings, likereference numerals refer to like features in the various views.

FIG. 1 is a cross-sectional view of a structure at an initialfabrication stage of a processing method in accordance with embodimentsof the invention.

FIG. 2 is a cross-sectional view of the structure at a fabrication stageof the processing method subsequent to FIG. 1 .

FIG. 3 is a cross-sectional view of a structure in accordance withalternative embodiments of the invention

DETAILED DESCRIPTION

With reference to FIG. 1 and in accordance with embodiments of theinvention, a structure 10 includes a waveguide core 12 positioned over adielectric layer 13, a substrate 14, and a grating 16 disposed in avertical direction between the waveguide core 12 and the substrate 14.In an embodiment, the dielectric layer 13 may be comprised of adielectric material, such as silicon dioxide, and the substrate 14 maybe comprised of a semiconductor material, such as single-crystalsilicon. In an embodiment, the dielectric layer 13 may be a buried oxidelayer of a silicon-on-insulator substrate.

In an embodiment, the waveguide core 12 may be comprised of a materialhaving a refractive index that is greater than the refractive index ofsilicon dioxide. In an embodiment, the waveguide core 12 may becomprised of a dielectric material, such as silicon nitride. In analternative embodiment, the waveguide core 12 may be comprised ofsilicon oxynitride. In an alternative embodiment, the waveguide core 12may be comprised of a semiconductor material, such as silicon. Inalternative embodiments, other materials, such as a polymeric materialor a III-V compound semiconductor material, may be used to form thewaveguide core 12. In an embodiment, the waveguide core 12 may be formedby patterning a layer of the material with lithography and etchingprocesses. In an embodiment, the waveguide core 12 may be formed bypatterning the single-crystal silicon device layer of asilicon-on-insulator substrate with lithography and etching processes.

In the representative embodiment, the waveguide core 12 is a ridgewaveguide core. In alternative embodiments, the waveguide core 12 may bea rib waveguide core, a slot waveguide core, or a different type ofwaveguide core. In embodiments, waveguide core 12 may include linearsections, curved sections, and/or tapered sections.

The grating 16 includes multiple layers 18 and multiple layers 20 thatare arranged in a layer stack in which the layers 20 alternate in avertical direction with the layers 18. The waveguide core 12 ispositioned on the dielectric layer 13 to overlap with the layers 18, 20of the grating 16. In an embodiment, the layers 20 may fully separatethe layers 18 from each other. Due to the alternating arrangement,adjacent pairs of the layers 18, 20 may define respective periods of thegrating 16. In an embodiment, the grating 16 may include five or moreperiods each including an adjacent pair of the layers 18, 20. In anembodiment, one of the layers 18 may be positioned adjacent to thedielectric layer 13. In an embodiment, one of the layers 18 may directlycontact the dielectric layer 13. In an embodiment, one of the layers 18may be positioned adjacent to the substrate 14. In an embodiment, one ofthe layers 18 may be in direct contact with the substrate 14.

In an embodiment, the layers 18 may be comprised of a material having arefractive index (i.e., index of refraction) that is greater than therefractive index of the material constituting the layers 20 to providean index contrast or variation. In an embodiment, the layers 20 may becomprised of a dielectric material, such as silicon dioxide, that has alower refractive index than the waveguide core 12. In an embodiment, thelayers 18 may be comprised of a material having a refractive index thatis greater than the refractive index of silicon dioxide. In anembodiment, the layers 18 may be comprised of a III-V compoundsemiconductor material, such as gallium nitride, aluminum nitride,indium nitride, or a combination of these materials. In an embodiment,the layers 18 may be comprised of gallium nitride. In an embodiment, thelayers 18 may be comprised of a Group IV semiconductor material, such assilicon. In an embodiment, the layers 18 may be comprised of siliconcarbide. In an embodiment, the layers 18 may be comprised of amorphoussilicon. In an embodiment, the layers 18 may be comprised of an opticalgain material, such as a material including indium nitride quantum dots.In an embodiment, the layers 18 may be comprised of a two-dimensional(2D) material, such as graphene, having a thickness of less than orequal to about 2 nanometers. In an embodiment, the layers 18 may becomprised of a nanostructured material, such as a layered superlattice.

In an embodiment, the grating 16 may be formed by multiple wafer-bondingprocesses that laminate the layers 18 onto the substrate 14 using thelayers 20 to promote bonding. In an alternative embodiment, the grating16 may be formed by sequentially depositing the layers 18 and the layers20 onto the substrate 14.

In an embodiment, the layers 18 may be planar films having a uniformthickness between a planar top surface and a planar bottom surface. Inan embodiment, the pitch and thickness of the layers 18 may be uniformto define a periodic arrangement having a periodic variation in theindex contrast. In an embodiment, the layers 18 may have a thickness ofabout 200 nanometers to about 700 nanometers.

With reference to FIG. 2 in which like reference numerals refer to likefeatures in FIG. 1 and at a subsequent fabrication stage, a dielectriclayer 22 is formed over the waveguide core 12. The dielectric layer 22may be comprised of a dielectric material, such as silicon dioxide, thatis deposited and then planarized following deposition. In therepresentative embodiment, the waveguide core 12 is embedded in thedielectric layer 22. The dielectric material constituting the dielectriclayer 22 may have a refractive index that is less than the refractiveindex of the material constituting the waveguide core 12.

A back-end-of-line stack 24 may be formed over the dielectric layer 22.The back-end-of-line stack 24 may include multiple dielectric layersthat are each comprised of a dielectric material, such as silicondioxide, silicon nitride, tetraethylorthosilicate silicon dioxide, orfluorinated-tetraethylorthosilicate silicon dioxide.

The grating 16 may provide efficient optical isolation from thesubstrate 14 for light propagating in the waveguide core 12, which maybe effective to reduce mode leakage to the substrate 14 and improveperformance. The grating 16 may permit the thickness of the dielectriclayer 13 to be reduced, which may be effective to reduce self-heating ofthe waveguide core 12 and which may allow electrical back biasing for anoptical component incorporating the waveguide core 12. The grating 16may also provide the ability to intentionally alter or tailor the modesize of light propagating in the waveguide core 12.

With reference to FIG. 3 and in accordance with alternative embodimentsof the invention, the pitch and/or the thickness of the layers 18 may beapodized (i.e., non-uniform) to define a non-periodic arrangement havingan aperiodic variation in the index contrast. The grating 16 withapodized layers 18 may, for example, provide sidelobe suppression inaddition to efficient optical isolation from the substrate 14.

The methods as described above are used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (e.g., as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. Thechip may be integrated with other chips, discrete circuit elements,and/or other signal processing devices as part of either an intermediateproduct or an end product. The end product can be any product thatincludes integrated circuit chips, such as computer products having acentral processor or smartphones.

References herein to terms modified by language of approximation, suchas “about”, “approximately”, and “substantially”, are not to be limitedto the precise value specified. The language of approximation maycorrespond to the precision of an instrument used to measure the valueand, unless otherwise dependent on the precision of the instrument, mayindicate a range of +/−10% of the stated value(s).

References herein to terms such as “vertical”, “horizontal”, etc. aremade by way of example, and not by way of limitation, to establish aframe of reference. The term “horizontal” as used herein is defined as aplane parallel to a conventional plane of a semiconductor substrate,regardless of its actual three-dimensional spatial orientation. Theterms “vertical” and “normal” refer to a direction perpendicular to thehorizontal, as just defined. The term “lateral” refers to a directionwithin the horizontal plane.

A feature “connected” or “coupled” to or with another feature may bedirectly connected or coupled to or with the other feature or, instead,one or more intervening features may be present. A feature may be“directly connected” or “directly coupled” to or with another feature ifintervening features are absent. A feature may be “indirectly connected”or “indirectly coupled” to or with another feature if at least oneintervening feature is present. A feature “on” or “contacting” anotherfeature may be directly on or in direct contact with the other featureor, instead, one or more intervening features may be present. A featuremay be “directly on” or in “direct contact” with another feature ifintervening features are absent. A feature may be “indirectly on” or in“indirect contact” with another feature if at least one interveningfeature is present. Different features “overlap” if a feature extendsover, and covers a part of, another feature.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration but are not intended tobe exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

1. A structure comprising: a substrate; a waveguide core; and a gratingdisposed in a vertical direction between the waveguide core and thesubstrate, the grating including a first plurality of layers and asecond plurality of layers that alternate in the vertical direction withthe first plurality of layers, the first plurality of layers comprisinga first material having a first refractive index, and the secondplurality of layers comprising a second material having a secondrefractive index that is greater than the first refractive index,wherein the first material is silicon dioxide, the second material isgallium nitride, the waveguide core comprises a third material having athird refractive index, and the first refractive index is less than thethird refractive index. 2-7. (canceled)
 8. The structure of claim 1further comprising: a dielectric layer positioned in the verticaldirection between the grating and the waveguide core.
 9. The structureof claim 8 wherein one of the second plurality of layers directlycontacts the dielectric layer.
 10. The structure of claim 9 wherein oneof the second plurality of layers directly contacts the substrate. 11.The structure of claim 1 wherein one of the second plurality of layersdirectly contacts the substrate.
 12. The structure of claim 1 whereinthe second plurality of layers have a uniform thickness.
 13. Thestructure of claim 1 wherein the second plurality of layers have anon-uniform thickness.
 14. (canceled)
 15. A structure comprising: asubstrate; a waveguide core; and a grating disposed in a verticaldirection between the waveguide core and the substrate, the gratingincluding a first plurality of layers and a second plurality of layersthat alternate in the vertical direction with the first plurality oflayers, the first plurality of layers comprising a first material havinga first refractive index, and the second plurality of layers comprisinga second material having a second refractive index that is greater thanthe first refractive index, wherein the second material is atwo-dimensional material.
 16. A structure comprising: a substrate; awaveguide core; and a grating disposed in a vertical direction betweenthe waveguide core and the substrate, the grating including a firstplurality of layers and a second plurality of layers that alternate inthe vertical direction with the first plurality of layers, the firstplurality of layers comprising a first material having a firstrefractive index, and the second plurality of layers comprising a secondmaterial having a second refractive index that is greater than the firstrefractive index, wherein the second material is a nanostructuredmaterial.
 17. The structure of claim 1 wherein the third material issilicon.
 18. The structure of claim 1 wherein the third material issilicon nitride. 19-20. (canceled)
 21. The structure of claim 8 whereinthe dielectric layer is a buried oxide layer of a silicon-on-insulatorsubstrate.
 22. The structure of claim 15 wherein the first material is adielectric material that is an electrical insulator.
 23. The structureof claim 15 wherein the second plurality of layers have a uniformthickness.
 24. The structure of claim 15 wherein the second plurality oflayers have a non-uniform thickness.
 25. The structure of claim 15further comprising: a dielectric layer positioned in the verticaldirection between the grating and the waveguide core.
 26. The structureof claim 16 wherein the first material is a dielectric material that isan electrical insulator.
 27. The structure of claim 16 wherein thesecond plurality of layers have a uniform thickness.
 28. The structureof claim 16 wherein the second plurality of layers have a non-uniformthickness.
 29. The structure of claim 16 further comprising: adielectric layer positioned in the vertical direction between thegrating and the waveguide core.